1. Field of the Invention
This invention relates to a radiation concentrator for a photovoltaic device.
2. Discussion of Prior Art
Light concentrators can reduce considerably the cost of electricity from photovoltaic (PV) cells. Unfortunately, conventional high concentration techniques require solar tracking, which can be expensive, and utilise only the direct component of radiation. In the late 1970""s a novel type of collector, the luminescent (or fluorescent) collector, was extensively investigated [see References 1, 2, 3], consisting of a transparent sheet doped with appropriate organic dyes. The sunlight is absorbed by the dye and then re-radiated isotropically, ideally with high quantum efficiency (QE), and trapped in the sheet by internal reflection. The trapped light is converted at the edge of the sheet by a PV cell operating at optimum efficiency with band-gap just below the luminescent energy. The excess photon energy is dissipated in the collector by the luminescent red-shift (or Stokes"" shift) rather than in the cell where heat reduces efficiency. Furthermore, a stack of sheets doped with different dyes [see Reference 2] can separate the light, as in FIG. 1 of the accompanying drawings, and cells can be chosen to match the different luminescent wavelengths.
The advantages over a geometric concentrator are that solar tracking is unnecessary, the material is cheap and both direct and diffuse radiation can be collected [see References 3, 4]. In addition, such concentrators are not limited by phase-space conservation i.e. Liouville""s Theorem [see References 5, 6], in contrast to geometric concentrators [see Reference 7].
The development of these known concentrators was limited by practicalities such as, firstly, the stringent requirements on the dye, namely high QE, suitable absorption spectra and red-shifts and stability under illumination [see References 5, 8]. The second limitation was the need for transparent host materials at the luminescent wavelengths and thirdly the absence of high efficiency PV cells of suitable band-gap. Concentration ratios of 10xc3x97 were achieved [see Reference 5]. A typical measured electrical efficiency with a two-stack concentrator with GaAs solar cells was 4% [see Reference 5], whereas the original predictions were in the range 13-23% [see Reference 2].
Viewed from one aspect the present invention provides a radiation concentrator for use with a photovoltaic device, said radiation concentrator comprising: a wave-guide containing a plurality of quantum dots, incident radiation upon said quantum dots being red-shifted by said quantum dots to form red-shifted radiation and said red-shifted radiation being internally reflected within said wave-guide to a wave-guide output, said wave-guide being formed of a material substantially transparent to said red-shifted radiation.
The use of quantum dots greatly improves the practicality of a luminescent solar concentrator. Certain properties of quantum dots, in particular their luminescent efficiency, tunability of absorption thresholds and size of red-shifts, make them good replacements for the organic dyes which limit the performance of this inexpensive, concentrator technology. Furthermore, the use of dielectric wave-guide technology and photo-voltaic cells, in particular the ability of quantum well cells to tune the band-gap, also mean that high overall efficiency is possible in solar and thermophotovoltaic applications.
A thermodynamic model may be used to show that red-shifts are determined by the spread of dot sizes. The model can also be used to improve concentrator performance.